1. Field of the Invention
The present invention relates to a pellicle having a filter for preventing impurities from entering through a vent for controlling atmospheric pressure which is provided on a side surface of a pellicle frame in order to prevent a swelling or a dent of a pellicle film due to change of external atmospheric pressure.
2. Description of the Related Art
In manufacture of semiconductor devices such as LSI, VLSI or the like or devices for a liquid crystal display, an exposure original plate it is used in the present specification as a general term for photo mask, reticle or the like) is placed on a semiconductor wafer such as silicon wafer or an original plate for liquid crystal, and is exposed to light to transfer a pattern with transmitted light. Namely, lithography is performed.
However, if impurities (hereinafter occasionally called trash, dust) are adhered to the exposure original plate in the above-mentioned step, the impurities absorb or bend light, so that a pattern transferred to the semiconductor wafer or the original plate for liquid crystal may be distorted, edge of the pattern may be rough, white portion may be dirtied to be black, resulting in degradation of size, quality, appearance or the like of the pattern. As a result, there are occurred problems of lowering of performance and production yield of a semiconductor device or a device for a liquid crystal display.
In order to avoid the above-mentioned problems, lithography is generally performed in a clean room. However, it is difficult to keep an exposure original plate completely clean even in a clean room. Accordingly, there has been adopted a method of attaching a pellicle that well transmits light for exposure on the surface of the exposure original plate in order to prevent impurities or the like from adhering to the surface of the exposure original plate.
As described above, if a pellicle is attached, impurities are adhered to a pellicle film, not directly to the surface of the exposure original plate. Accordingly, when focusing on a pattern of the exposure original plate during lithography, presence of such impurities do not affect on transfer, and therefore the above-mentioned problems are not caused.
Since the above-mentioned pellicle is provided in order to prevent impurities (trash) from adhering to the exposure original plate, after the pellicle is attached to the exposure original plate, the pellicle is generally closely adhered to the exposure original plate so that impurities outside of the pellicle may not enter the pellicle. Accordingly, there is no ventilation between outside and inside of the pellicle, and thus space formed by the exposure original plate and the pellicle is airtight space.
Although the pellicle is generally used under atmospheric pressure, it is sometimes affected by atmospheric pressure depending on height at which the pellicle is used, season, change in climate. Furthermore, it is sometimes used under decreased pressure.
If atmospheric pressure changes as described above, in the state that the pellicle is attached to the exposure original plate, the pellicle film is swelled or dented, since the space formed by the exposure original plate and the pellicle is airtight space as described above, resulting in change in height of the pellicle film. As a result, for example, the pellicle film is swelled, so that it is brought into contact with a part of an apparatus for exposure. Furthermore, when inspection of impurities on the pellicle film is conducted, for example, focus of a laser for illuminations is shifted, resulting in misdetection. They are severe problems on use.
In these cases, a vent will be effective for controlling atmospheric pressure of inside and outside of the pellicle. However, if a vent is merely provided, impurities will enter with inflow of air through the vent. As a result, original purpose of the pellicle that is preventing impurities from adhering to the surface of the exposure original plate inside of the pellicle cannot be achieved.
From such a point of view, there was proposed a method of controlling atmospheric pressure of inside and outside of the pellicle with preventing impurities from entering, by providing a vent on the side surface of the pellicle frame, and also providing a filter outside of the vent (See Japanese Utility Model publication No. 63-39703).
In such a method of providing a small vent and adhering a filter to the outside thereof, a resin plate or a metal plate wherein a lot of pores having a diameter equivalent to or less than an assumed diameter of impurity is generally used as a filter, in order to prevent impurities having a size causing trouble when conducting lithography from entering.
Meanwhile, the surface and inside of the filter is likely to be damaged, for example, by bending or rubbing or the like, when the filter is adhered to the pellicle frame, resulting in generation of the impurities. Furthermore, impurities which has been existing potentially in the fine pores of the filter since the filter was manufactured are sometimes released to inside of the pellicle due to airflow (ventilation) formed by difference of atmospheric pressure of inside and outside. In order to solve the problem, it is proposed to apply an pressure sensitive adhesive on a part of the filter to control release of the impurities (See Japanese Patent Application Laid-open (KOKAI) No.9-68792, No.9-160223).
However, actually, pattern rule of LSI has been getting finer as sub half micron, and a size of impurity that is allowable to adhere to the pattern surface of the mask has been getting finer. In such a circumstance, more cleanliness is required on use of pellicle. If impurity-trapping performance of a filter is pursued, there has been a problem that ventilating performance of the filter is lowered, so that a sufficient performance in atmospheric pressure controlling function cannot be achieved. On the contrary, if ventilating performance of the filter is pursued, impurity-trapping performance of the filter is relatively lowered, so that there will be caused contradiction that impurity will be allowed to enter the space formed by a pellicle film and an exposure original plate.
Meanwhile, air blow or the like from outside of the filter is sometimes conducted on the purpose of removing impurities. Since airflow passing through the filter by such air blow is far more violent than airflow involved by change in atmospheric pressure, impurities cannot be trapped and fixed sufficiently under conventional condition of treatment with a pressure sensitive adhesive, and impurities are sometimes released into the pellicle.
The present invention has been accomplished to solve the above-mentioned previous problems. The first object of the present invention is to provide a pellicle with a filter having both of impurity-trapping performance and ventilating performance, and a method for producing it.
The second object of the present invention is to provide a pellicle with a filter having ventilating performance wherein impurities can be fixed without being released even under violent airflow such as air blow or the like and finer impurities can be trapped, with keeping ventilating performance.
In order to achieve the above-mentioned first object, the present invention provides a pellicle having a pellicle frame with at least one vent for controlling atmospheric pressure and a filter that is attached so as to cover the vent, wherein it takes 5 minutes to 180 minutes, to restore the pellicle film swelled during a step of attaching the pellicle to the exposure original plate under atmospheric pressure of 760 mmHg followed by reducing atmospheric pressure to 500 mmHg and keeping the pressure, to the original state.
As described above, if a measurement method and a measurement condition for appreciating ventilating performance of the filter are defined, and a pellicle with a filter is produced so that the results of the measurement fill the condition, namely the condition wherein it takes 5 minutes to 180 minutes, to restore the pellicle film that is swelled, the impurity-trapping performance of the resultant pellicle is significantly improved, so that not only impurities from outside, but also impurities potentially existing inside of the filter due to airflow generated by difference of atmospheric pressure of inside and outside are surely trapped by static electricity formed by friction between airflow and resin or by fine pores, and are hardly released into the pellicle. Furthermore, practical atmospheric pressure controlling function of the pellicle is sufficiently ensured by such ventilating performance. Accordingly, the above pellicle with a filter can have both of ventilating performance and impurity-trapping performance, and is well balanced.
Preferably, the pellicle of the present invention relates to a pellicle wherein a filter is treated with a resin. Preferably, the resin for treating the filter is tacky. In the pellicle with a filter of the present invention, the filter treated with a resin is preferably produced by coating the surface of the filter with a liquid resin and/or by immersing the filter in a liquid resin.
As described above, if the filter is treated with resin, impurity-trapping performance can be improved without lowering the ventilating performance.
In order to achieve the above-mentioned second object, the present invention provides a pellicle with a filter having a pellicle frame with at least one vent for controlling atmospheric pressure wherein all over the inner surface of the filter attached so as to cover said vent is impregnated with a resin, and 50% by volume or more of the filter is impregnated therewith.
As described above, since the filter wherein all over the inner surface of the filter attached so as to cover the vent is impregnated with a resin, and 50% by volume or more of the filter is impregnated with the resin is attached to the vent for controlling atmospheric pressure provided in the pellicle frame, impurities that have been existing potentially in the fine pores since the filter was manufactured can be surely fixed so as not to be released, and are hardly released even with violent air blow. Furthermore, the impurities generated in the filter or entering from the outside are trapped by static electricity generated due to friction between airflow and resin or by fine pores, and hardly enter the pellicle. Furthermore, practical ventilating performance of the filter is sufficiently ensured also by resin impregnation.
In this case, the resin with which the filter is impregnated is preferably tacky.
As described above, if the filter is impregnated with a tacky resin, impurity-trapping performance and fixing performance can be further improved.
In the pellicle with a filter of the present invention, the filter impregnated with the resin is preferably produced by coating the surface of the filter with a liquid resin and/or by immersing the filter in a liquid resin.
As described above, if the filter is impregnated with the resin by coating the surface of the filter with a liquid resin and/or by immersing the filter in a liquid resin, all over the surface of fine pores of the filter can be coated uniformly, so that it is possible to surely fix impurities which have been existing potentially since the filter was produced, and released by strong air blow, or impurities generated when the filter is broken by external pressure, and to trap the impurities entering from the outside surely, in the pellicle with the filter.
The present invention also relates to a method for producing a pellicle with a filter having a pellicle frame in which at least one vent for controlling atmospheric pressure is provided, wherein a filter having a pore size defined so that it takes 5 minutes to 180 minutes to restore the pellicle film swelled during a step of attaching the pellicle to the exposure original plate under atmospheric pressure of 760 mmHg followed by reducing atmospheric pressure to 500 mmHg and keeping the pressure, to the original state, is attached so as to cover said vent.
In order to produce the pellicle with the filter as described above, it is necessary to determine experimentally in detail a relation between a pore size of the filter, a restoring time of the film and impurity-trapping performance (for example, number of impurities increased in the pellicle during ventilating performance test), to select the filter having a pore size corresponding to a restoring time (ventilating performance) and impurity-trapping performance to be required, and to attach it to the pellicle frame so as to cover the vent.
As described above, according to the present invention, there can be provided a pellicle with a filter having both impurity-trapping performance that both of impurities entering from the outside of the filter and impurities generated inside of the filter can be trapped and fixed almost completely and practically sufficient ventilating performance that is atmospheric pressure controlling function.
Furthermore, according to the present invention, in the pellicle having a pellicle frame with at least one vent for controlling atmospheric pressure, it is possible to fix impurities which have been existing potentially since the filter was produced, and thereby prevent them from being released by violent airflow such as air blow or the like, without lowering ventilating performance of the filter provided for preventing impurities from entering through a vent. Even if they become fine impurities and try to enter the pellicle, they can be surely trapped and fixed. Furthermore, there can be formed a filter wherein a swelling of a pellicle film due to change in atmospheric pressure and a restoring speed do not hinder actual use.
Accordingly, if photolithography is conducted using a pellicle with a filter of the present invention, regular transfer pattern can be obtained, so that performance and production yield of a semiconductor device or a device for a liquid crystal display can be improved.